[llvm] [MC][x86] Fix missing check in MC binary expression (PR #75693)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

Thanks for the fix! I think it would be a typo when added the code in https://github.com/llvm/llvm-project/commit/7fd992a75502b7eb251daa1b1e4d23a73fa76d53
But I'd like to invite @rnk to double confirm it.

https://github.com/llvm/llvm-project/pull/75693